As a prior art for finishing a gear surface of a bevel gear, U.S. Pat. No. 3,740,899 discloses a method, as shown in FIG. 7, where the bevel gear is rocked along arrow 30 to perform a lap finishing. The rocking direction of the engaging gear teeth is shown by arrow 30'.
On the other hand, U.S. Pat. No. 2,904,934 discloses another sample of lap finishing where, as shown in FIG. 8, the bevel gears are rocked along the gear trace direction 31'.
In both of the above prior art patents, the rocking motion is not generated in a manner such that it has a pivot center which is a pitch apex 19 of a base pitch circle cone. Thus, the finished product is not finished to a high tolerance and also has the disadvantages of higher noise and lower productivity.